Wednesday, November 13, 2013

Debris flow down the wall of Dugan J

Southeast wall of Dugan J, where material flowed downward and came to rest at the base of the crater wall. Field of view is approximately 2 km across. From LROC Narrow Angle Camera (NAC) mosaic M1131216329LR, an oblique observation swept up during LRO orbit 18833, August 15, 2013; resolution roughly 3 km per pixel [NASA/GSFC/Arizona State University].

H. Meyer
LROC News System

The farside crater Dugan J (roughly 13 km in diameter, 61.458°N, 107.898°E) is located northeast of Compton crater and well east of the marginally near-side Mare Humboldtianum.

Dugan J is a fresh, simple crater, which is why it appears crisply bowl-shaped and with steeply sloping walls in the LROC NAC oblique image, further below.

In that mosaic, notice the subtle surface expression of a filled crater in the foreground of the oblique image. This nearly 23 km diameter crater is almost unrecognizable because it is filled to the brim with ejecta from nearby impacts, including Dugan J. That's a lot of ejecta!

In the opening image, low-albedo material rests on the southeast wall. Unlike flows of liquid impact melt in craters, the flow observed above is composed of fine-grained, granular debris originating from the crater walls that acted like a fluid as it was pulled downslope by gravity.

Granular debris flows are common in fresh craters, where the walls are steeply sloping, promoting downhill movement of eroded material from high up on the crater walls to the crater floor. Over time, the walls degrade and shallow out. When their slopes reach the angle of repose (for the Moon: near 30° from horizontal), it becomes more difficult to move material downslope. However, if the slopes are disturbed by forces in addition to gravity, such as seismic shaking from a nearby impact, material can still be mobilized.

LROC Wide Angle Camera (GLD100-WAC) mosaic showing Dugan J in context. The field of view from which the LROC Featured Image was cropped is designated by the yellow, dotted quadrangle, and two filled near-ghost craters are circled by orange circles. The black ellipse denotes the location of the Compton-Belkovich (Th anomaly) volcanic complex. Compton crater is approximately 164 km in diameter [NASA/GSFC/Arizona State University].

The granular flow appears to have originated from near the rim of the crater, where low-albedo material can be seen streaking the high-albedo crater wall. There is also some wall material external to the flow in Today's Featured Image that has been degraded and has started to cover part of the floor with rubble. The floor of Dugan J is covered in impact melt and blocks that are being worn into boulders.